The present invention is a combined rope and fish net guard and it is particularly adapted for use in large ocean going marine vessels. These vessels are commonly equipped with a rope guard that extends outwardly from the stern frame of the vessel to the propeller hub, thereby protecting the propeller shaft from stray lines, seaweed, nets, and other debris that may be encountered in the shipping lanes through which these vessels pass.
The advent of the mono-filament fish nets, however, has created a new hazard for large ocean going vessels. These mono-filament fish nets, some of which are hundreds of yards in length, may be found drifting in the ocean passages and in certain parts of the world, are staked out with little regard for commercial shipping lanes or the channel markers which guide these vessels. While the horsepower of the vessel's engines, and the size of the propellers render the fish nets a minimal threat to fouling the vessel's propellers, the extreme fineness of the mono-filament net renders it capable of posing still an additional threat to the vessel shaft seals. As the net becomes fouled in the propeller, it is stretched out and drawn across the conventional rope guard, and may be drawn through the gap between the stationary rope guard, and the rotating propeller, even though the gap is as small as a few millimeters in width. Once through the rope guard, the net will become wrapped around the vessel propeller shaft, and will work its way into the stern shaft seal of the vessel. In this position, it may abrade the shaft seal, or cause the shaft seal to leak, thereby presenting a potential and substantial threat to the vessel.
Depending on the direction of the vessel's motion, the RPM of the shaft, and the oil pressure maintained in the shaft seal, the netting may enable sea water to contaminate the oil used to lubricate the main stern bearings, or if the pressure in the oil system is higher than the sea water pressure, the nettings may allow the oil to escape into the shipping lane, or the harbor in which the vessel is maneuvering.
If the oil leaks out, it may contaminate the surrounding environment, and result in a large fine for the vessel's owners, and possible catastrophic damage to the vessel if insufficient oil remains for the rear stern bearing. If sea water is allowed in, it may cause a catastrophic damage to the main stern bearing since the sea water will impair the lubricating efficiency of the oil system.
Further, given the size of the ocean going vessel, the captain is frequently unaware that the shaft has been fouled by a fish net until the net has already impaired the seal. The first indication of the problem to the captain is normally the discharge of the oil into the water, or the admixture of the sea water to the bearing oil.
When the shaft seal has been thus fouled by a fish net, the vessel must be stopped, a diver found, the rope guard opened, and the voyage delayed for several hours while the diver clears the netting from the shaft. In some case, where the seal has been damaged, or portions of the netting have been irretrievably wrapped between the shaft and the seal, it is necessary to remove or replace the shaft seal, thereby resulting still further delays, and great expense since the shaft seals on the vessel of this size can easily be several feet in diameter. The cost of keeping a fully loaded vessel idle can easily involve a loss in revenue and penalties of thousands of dollars for each hour that the ship is delayed.
Prior art rope and net guards have been designed primarily for smaller boats and outboard motors, wherein a line or net will actually foul and stall the propeller.
U.S. Pat. Nos. 4,447,215; 4,507,091; and 4,544,363 are illustrative of a line and net guard which uses a stationary blade and a rotating blade to sever lines and nets which might otherwise foul the propeller. While efficient, such devices would be inapplicable to a large going vessel, since scaling them up in a size necessary to make them practical, would result in an undue drag and disturbance of the water passing through the propeller.
U.S. Pat. No. 4,450,670 illustrates a stationary blade intended to be mounted adjacent to the propeller of an outboard motor. The blade is presented with three cutting edges to shear underwater foliage that might otherwise foul the propeller.
U.S. Pat. Nos. 67,982 and 1,649,657 disclose devices for preventing the fouling of a ship or a large vessel propeller. The '982 patent discloses a pair of rotating cutting rings which are mounted on the propeller shaft. The '657 patent discloses a series of serrated cutting edges which are stationary, and cooperate with the ship's propeller to sever any lines which are wrapped about the cutting edges by virtue of the motion of the ship's propeller. In one embodiment of this invention, axially extending teeth are provided on the inner surface of the guard for severing any ropes or lines which may find the way inside the guard. Guards of these designs also create undue drag and turbulence for the vessel's propellers